This invention relates to the aging of cathode ray tubes, and more particularly relates to an improved aging process in which dark center cathodes caused by the aging of the focusing electrode are substantially reduced.
In the manufacture of cathode ray tubes for television and other display applications, various tube processing steps are carried out to insure an acceptable life of reliable operation in the field. This processing begins after assembly of the tube components, and includes: exhausting and baking the tube to evacuate the envelope and outgas the tube components; flashing a getter onto the internal surfaces of the tube and components to provide continuous gettering of residual contaminants which are outgassed during tube operation; activating the cathodes of the electron gun by heating to promote the formation of low work function species in the emission layer; aging the cathode and lower grid elements of the gun to maintain cathode activation; and finally high voltage conditioning of the electron gun to remove particles and projections which could lead to interelectrode arcing.
The rate of outgassing is time and temperature dependent, and the throughput demands of the manufacturing process as well as the limited thermal stability of certain tube components make complete outgassing during exhausting and baking impractical. Thus, some residual gas and gas-producing contaminants, such as hydrocarbons, remain in the tube after sealing of the exhaust tubulation.
Getter flashing usually introduces additional hydrocarbon contaminants into the tube. These hydrocarbons cannot be effectively adsorbed by the non-bakable barium getters commonly employed in many types of color television picture tubes. However, during subsequent aging, these hydrocarbons are dissociated into getterable components, resulting in the reduction of residual gas in the tube to acceptable levels.
Unfortunately, the aging process has also been found to result in a condition known as "dark center cathode", which by analysis has been found to be due to a carbon deposit in the center of the emissive layer of the cathode. Surprisingly, this deposit does not materially reduce cathode emission. However, it does restrict emission to the area of the perimeter of the emissive layer, resulting in a hollow beam which interferes with proper focusing and image resolution at the screen.
In U.S. Pat. No. 4,457,731, the dark center cathode problem is addressed for reprocessed cathode ray tubes. Tubes rejected for gun-related defects can be salvaged by "regunning", i.e,. replacing the defective gun with a new one. Since this regunning operation necessarily reopens the envelope to the ambient, the tube must be reprocessed. The patent teaches that dark center cathodes can be substantially reduced by flashing the getter after aging, so-called "post-flashing". However, when post-flashing is practiced on virgin tubes, unacceptably high gas levels result.
Accordingly, it is an object of this invention to provide an improved method for processing cathode ray tubes which reduces the incidence of dark center cathodes and does not result in unacceptably high gas levels.
It is another object of this invention to provide an improved aging process which may be carried out after sealing and getter flashing without producing dark center cathodes, and which will reduce residual gas to an acceptable level.